Friction stir tool

ABSTRACT

A friction stir tool excellent in productivity, high temperature strength, and wear resistance at high temperatures. The friction stir tool is formed of a Co-based alloy comprising crystal grains containing a γ′ precipitate phase dispersed and precipitated therein, and a crystal grain boundary region and a precipitate phase between adjacent crystal grains, in which the precipitate phase is at least one phase selected from a μ phase, a Laves phase and a carbide phase.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent applicationserial No. 2009-215998, filed on Sep. 17, 2009, the content of which ishereby incorporated by reference into this application.

This application is a Continuation application of prior application Ser.No. 12/805,832, filed Aug. 20, 2010, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a friction stir tool.

2. Description of Related Art

There exists a friction stir welding method which conducts welding byinserting a cylindrical member formed of a material which issubstantially harder than materials to be joined (hereinafter thecylindrical member is referred to as rotational tool, stirring tool orfriction stir tool. Or it is also referred to simply as welding tool.)into a joined part of the materials to be joined while rotating thecylindrical member, rotating and moving this rotational tool at the sametime to join the materials to be joined by the frictional heat generatedbetween the rotational tool and the materials to be joined.

Japanese Patent No. 2712838 (Patent Document 1) discloses a frictionwelding method for joining workpieces (1A, 1B) in a joint region (2), inwhich a probe (3) of material harder than the workpiece material isinserted into the joint region (2) and opposed portions of theworkpieces on both sides of the joint region to cause relative cyclicmovement between the probe and the workpieces so that frictional heat isgenerated to cause the opposed portions to take up a plasticizedcondition; the probe (3) is removed; and the plasticized portion isallowed to solidify and join the workpieces together, whereby theworkpieces are joined without relative movement between themselves. Thisjoining method involves softening the materials to be joined by thefrictional heat between the rotational tool and the materials to bejoined and utilizing the plastic flow phenomenon caused by the rotationof the rotational tool, and is based on a principle different from amethod of welding by dissolving the materials to be joined, for example,arc welding and the like.

JP-A-2008-36664 (Patent Document 2) discloses a friction stir weldingtool which has a cylindrical body of revolution and a probe coaxiallyprojecting from a shoulder on its end face, and joins a pair of joinedmembers by rotating the probe and inserting it into the joined parts ofthe joined members where they are abutted so that the joined part issoftened by the frictional heat generated and stirred, in which theprobe and the body of revolution are attachable and detachable, and theprobe is formed of a cemented carbide or a cobalt-based alloy copper(MP159).

JP-A-2005-152909 (Patent Document 3) discloses a rotational tool for afriction stir welding for integrating a joined part of metal pieces by afriction stirring, in which a deposition prevention film for preventingdeposition of a joining material metal on a surface of a portion whichcomes into contact with the metal.

JP-A-2004-82144 (Patent Document 4) discloses a friction stir weldingtool which is made of a material harder than a workpiece made of metaland is rotated, pressed and inserted into an abutment portion of a pairof workpieces made of the metal to join the workpieces made of the metalby friction stirring, the welding tool having a central member formed ofmetal, and a ceramic member which covers a region of the central memberwhich comes into friction contact with at least the workpieces made ofthe metal, the ceramic member comprising a nitride of Si, and thecentral member being a heat-resistant alloy comprising at least one ofFe, Ni, Co and W as a main component.

JP-A-2003-532543 (Patent Document 5) discloses a friction stir weldingtool which is capable of joining metal matrix composites (MMCs),ferroalloys, nonferrous alloys and superalloys by friction stir welding,the welding tool comprising a shaft, a shoulder, a pin, and a highlywear resistant material disposed in at least a portion of the shoulderand the pin, the shoulder being mechanically fixed on the shaft toprevent rotary motion of the shoulder with respect to the shaft, thehighly wear resistant material having a first phase and a second phase,and being produced at an ultra high temperature under ultrahigh-pressure, functionally allowing friction stir welding of MMCs,ferroalloys, nonferrous alloys and superalloys.

JP-A-2006-320958 (Patent Document 6) discloses a friction stir weldingtool which is capable of conducting friction stir welding on a metal oran alloy having a melting point of 1600° C. or higher as a workpiece,and at least a portion of the welding tool which is brought into contactwith the workpiece comprising iridium as a main component and rhenium,ruthenium, molybdenum, tungsten, niobium, tantalum, zirconium or hafniumor two or more of these as accessory components, and having a MicroVickers Hardness of 200 Hv or higher.

JP-A-2005-199281 (Patent Document 7) discloses a friction stir weldingtool having a probe pin extending from a tip surface of a rotatingrotator which is press-fitted onto a joined part of joined members andmoved along this joined part to join the joined members in the joinedpart by friction stir welding, in which the portions of the probe pinand rotator which come into contact with at least the above-mentionedjointed members are constituted by a WC-based cemented carbidecontaining Co in an amount of 5 to 18% by weight.

WO2007/032293 (Patent Document 8) discloses Co-based alloy having highlyheat resistant and high strength, comprising 0.1-10% of Al, 3.0-45% ofW, and Co as the remainder except for inevitable impurities in terms ofmass proportion, and has an L1₂ type intermetallic compound containing[Co₃(Al,W)] precipitated therein.

SUMMARY OF THE INVENTION

The friction stir tool of the present invention is characterized in thatit is formed of a Co-based alloy comprising crystal grains containing aγ′ precipitate phase dispersed and precipitated therein, crystal grainboundary region and precipitate phases between the adjacent crystalgrains, and that the precipitate phase is at least one phase selectedfrom a μ phase, a Laves phase and a carbide phase. In particular, it ispreferable that the γ′ precipitate phase has the composition: Co₃(Al,W),or (Co,X)₃(Al,W,Z). Here, X includes mainly Ni, and Z includes mainly Crand Ta.

According to the present invention, production costs can be reduced bycast and processing into various tool configurations can be readilyconducted. Therefore, the rotational tool for friction stir welding withexcellent productivity and a low degree of wear at high temperature canbe provided. In addition, by using the above welding tool, hard-to-weldmaterials such as Ti and Zr can be readily joined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view showing a constitution of a rotationaltool of an example according to the present invention.

FIG. 2 is a photograph of an appearance of an ingot of a Co-based alloywhich is a material of a rotational tool of the present invention.

FIG. 3 is an electron micrograph showing a microstructure of an ingot ofa Co-based alloy of an example according to the present invention.

FIG. 4 is a photograph showing a joined part of an SS400 steel materialproduced by using the rotational tool made of the Co-based alloy of thepresent invention.

FIG. 5A is a photograph of an appearance of the surface of therotational tool made of the Co-based alloy of the present inventionafter being used for welding (twice of 360 mm in length).

FIG. 5B is a photograph of an appearance of the surface of therotational tool made of the Co-based alloy of the present inventionafter being used for welding (37 times of 360 mm in length).

FIG. 6 is a photograph showing an appearance of a butt welding portionof a Ti-15V-3Cr-3Sn-3Al material produced by using the rotational toolmade of the Co-based alloy of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a friction stirring tool (a frictionstirring welding tool) for joining or reforming a material by frictionstirring.

An object of the present invention is to provide a friction stir toolhaving high temperature strength, excellent wear resistance at hightemperatures, and excellent productivity.

Friction stir welding of low-melting-point metallic materials such asaluminum alloys has already been put to practical use. Although thefriction stir welding of high-melting-point materials such as steelmaterials has been reported relatively early, the number of researchreports of the friction stir welding on the steel materials is muchfewer than those on the aluminum alloys.

One of reasons for such a few reports is that the joining is not easysince the welding tool is thermally deformed when the steel rotationaltool which is generally used in joining the aluminum alloys is appliedto joining steel materials. The characteristics required for a materialof the rotational tool in the friction stir welding of thehigh-melting-point materials such as steel materials include hightemperature strength, wear resistance and unreactive property.

Patent Document 5 describes a ceramic material such as PCBN(Polycrystalline Cubic Boron Nitride manufactured by MegaStir, US) as amaterial of the rotational tool suitable for joining the steel materialsand the high-melting-point materials.

Ceramic materials generally have high strength at high temperature, butare difficult to be cut at room temperature and cannot be easilyprocessed into various tool configurations. In addition, they remain tobe improved in terms of productivity since they are produced bysintering.

Meanwhile, Co-based alloys (cobalt-based alloys) also have relativelyhigh strength at high temperature, and are expected to be good materialsfor the rotational tools. Possibilities of applying Co alloys torotational tools are described in Patent Documents 2-4, 6 and 7 etc.

Since the Co-based alloys have relatively high machinability at roomtemperature compared with the ceramic materials, they can be processedinto various configurations relatively easily. Therefore, the materialsexpectedly have economic advantages. However, since the strength of theCo-based alloy mentioned above is abruptly lowered at a temperaturehigher than 800° C., deterioration of the material of the welding toolitself progresses, causing wear and damage of the welding tool when theCo-based alloy is used as a rotational tool for joining high-meltingpoint materials such as steel materials.

Patent Document 8 describes a Co-based alloy which seems to be capableof suppressing a rapid reduction in strength even at a high temperaturearound 1000° C. (a general joining temperature in the friction stirwelding of the steel material). Patent Document 8 also describes aγ′-strengthened Co-4Al-26.9W (mass %) ternary alloy, in which the γ′solid solution forming temperature is raised as the amount of Nicontained increases. The γ′ solid solution forming temperature is themost important factor to maintain high temperature strength.

The characteristics required for the stirring tool also include wearresistance, in addition to the high temperature strength. Meanwhile, asfor the Co-based alloy described in Patent Document 8, factors whichaffect the wear resistance are unclear. Possible factors include notonly γ′ solid solution forming temperature but also the amount of γ′(volume fraction) and other precipitates.

Problems of the wear resistance in the rotational tool mentioned abovecan be solved by using a Co-based alloy as a material of the rotationaltool. In the Co-based alloy, 10% to 80% by volume fraction of the γ′phase of Co₃(Al,W) with the L1₂ structure are caused to precipitate.

The reason why the volume fraction of the γ′ phase is limited to 10% to80% is as follows:

When the volume fraction of the γ′ phase was less than 10%, the hightemperature strength could not be maintained and therefore the frictionstir tool was partially deformed and the wear resistance was alsoinsufficient. Meanwhile, when the volume fraction of the γ′ phase wasmore than 80%, β phase with B2 structure and other phases wereexcessively precipitated, and the wear resistance of the tool degraded.

In addition to the characteristics of the Co-based alloy mentionedabove, it is preferable that the Co-based alloy has characteristics thatone or more phase (at least one phase) selected from a μ phase, a Lavesphase and carbide (also referred to as a carbide phase.) areprecipitated.

In a stirring temperature range of the friction stirring, a desirablechemical composition of the Co-based alloy which can maintain the hightemperature strength and the wear resistance is such that about 10 at. %of Al, about 7.5 at. % of W, about 3 at. % of Ta, about 10 at. % of Cr,about 0.06 at. % of B, and about 0.6 at. % of C are contained, and Ni iscontained in an amount of 30 to 50 at. %. In addition, it is desirablethat Co is contained in an amount of 25 at. % or more. Herein, at. %, aunit which can be also indicated as atomic % (atomic percent),represents the number of each constituent atom with respect to thenumber of all atoms which constitute the alloy.

The proof strength of the Co-based alloy at room temperature in theabove-mentioned composition range is about 800 MPa, which allowsrelatively easy machining. In addition, since the Co-based alloy can beprocessed by precision casting, it can be determined to have a largeeconomic effect on production of a welding tool.

The friction stir tool of the present invention is characterized bycomprising a cylindrical shank, a shoulder formed in an end portion ofthis shank, and a pin formed in an end portion of this shoulder. Thefriction stir tool is preferably an integrally formed article producedby casting or a machined product produced by using the Co-based alloy.The friction stir tool of the present invention is used by being mountedon a friction stir welding apparatus.

Examples of the present invention will be described below with referenceto drawings.

Example 1

FIG. 1 is an example of the tool constitution in the rotational tool ofthe present invention.

A friction stir tool 101 is integrally machined from an ingot producedby precision casting. Rigidity of the welding tool can be improved byintegrally processing the friction stir tool 101, and simplification ofthe welding tool production can be achieved, allowing significant costreduction.

The friction stir tool 101 is constituted by a shank 102 connected tothe main shaft of a welding apparatus, a shoulder 103 which comes intocontact with the surface of materials to be joined when joining, and apin 104 inserted into the materials to be joined when joining.

The shape of the welding tool 101 used was such that the diameter of theshoulder 103 was 15 mm. The pin 104 had a diameter in a pin-shoulderconnecting part 105 connected to the shoulder 103 of 6 mm, while it hada diameter of 3.5 mm at a pin tip 106. The length of the pin 104 wasadjusted to 1.8 mm. The thickness of an iron-based material used was 4mm, which is a material to be joined. As the joining conditions, therotational speed and joining speed of the welding tool 101 used were 250rpm and 500 mm/min, respectively.

FIG. 2 is a photograph of the appearance of an ingot of the Co-basedalloy produced by the precision casting.

The ingot was produced to have the composition ofCo-40Ni-10Al-7.5W-3Ta-10Cr-0.06B-0.6C. The units are all at. % (atomic %(atomic percent): the number of each constituent atom with respect tothe total number of atoms constituting alloy). The amount of Cocontained is about 29%. The outer diameter of the ingot was about 30 mm,and its length was about 100 mm, excluding a riser portion. It is a sizesufficient for producing the rotational tool for friction stirring. Inaddition, it was confirmed that the ingot could be produced in anextremely short time of about a few minutes, and had excellentproductivity. It was also confirmed that a welding tool having a nearnet shape could be produced by the precision casting.

After the ingot was produced, it was heat-treated at 1250° C. andair-cooled, followed by a heat-treatment at 1000° C. This is forprecipitating a γ′ phase of (Co,X)₃(Al,W,Z) with the L1₂ structurenecessary to maintain high temperature strength. Herein, X is mainly Ni,and Z is mainly Cr and Ta. In addition, the temperature of theabove-mentioned heat treatment is also effective in precipitating a μphase, a Laves phase and carbide, which are effective for enhancing thewear resistance and the grain boundary region.

FIG. 3 shows a microstructure photograph of the ingot.

The ingot is composed of a crystal grain 301, a γ′ precipitate phase 302finely dispersed and precipitated in a γ phase with an A1 structurewithin a grain of the crystal grain 301, a crystal grain boundary region303, and a precipitate phase 304 existing at the crystal grain boundaryregion 303. The volume fraction of the γ′ precipitate phase 302 is about60%. The precipitate phase 304 is one or more types of phases selectedfrom a μ phase, a Laves phase and a carbide phase. The precipitate phase304 is a component which stabilizes the grain boundary region (a grainboundary region stabilizing component). Herein, The crystal grain 301 isa single grain which is surrounded by the crystal grain boundary region303, in which a number of the γ′ precipitate phase 302 (γ′ grains) aredispersed in a Co matrix.

Such a microstructure is effective in improving the high temperaturestrength and the wear resistance.

Joining was conducted on iron-based material formed of SS400 by using afriction stir tool produced by processing the ingot shown in FIG. 2.

FIG. 4 is a photograph of the appearance of a joined part of the SS400steel material (iron-based material) joined by using a friction stirtool produced by processing the ingot shown in FIG. 2 into the shapeshown in FIG. 1.

As shown in this FIG. 4 (the joined part of the SS400 material mentionedabove), it was confirmed that the friction stir welding of the SS400material mentioned above by using the friction stir tool of the presentinvention was possible.

The rotational speed and joining speed of the welding tool 101 used asfor joining conditions were 250 rpm and 500 mm/min, respectively. Thejoining distance was 360 mm, and joining was conducted multiple times.Changes in the shape of the friction stir tool due to joining distancewere observed to evaluate the performance of the friction stir tool.

FIG. 5A shows the appearance of the surface of the welding tool afterjoining for a distance of 360 mm twice and FIG. 5B shows the appearanceof the welding tool after joining for 37 times. The total joiningdistance of the two trials was 0.72 m and 13.32 m, respectively.

As can be seen from FIGS. 5A and 5B, there is no visible damage found inthe pin and the shoulder even after joining for a distance of 13.32 m(37 times). Almost no wear was observed, although the changes in theshape of the welding tool after joining were not clearly determinedbecause the jointing material was slightly deposited on the pin and theshoulder.

Durational life of the welding tool was evaluated by using the abovewelding tool. As a result, it was confirmed that the degree of wear ofthe welding tool was low even after joining the total joining distanceof 45 m or longer, and that the welding tool could be used as a weldingtool.

The results described above demonstrate that the Co-based alloy tool canprovide a friction stir tool for the SS400 steel material with a lowdegree of deformation and wear.

Examples 2 to 4

The finding that the friction stir welding by using the friction stirtool 101 of FIG. 1 can be applied to pure titanium and titanium alloysin addition to the iron-based materials was obtained. The titaniummaterials used in these examples are industrial pure Ti (Example 2),titanium alloy Ti-6Al-4V (Example 3), and titanium alloyTi-15V-3Cr-3Sn-3Al (Example 4).

As for joining conditions, the rotational speed and joining speed of thewelding tool 101 used were 200 rpm and 100 mm/min, respectively.

FIG. 6 is a photograph of the appearance of an abutment portion of theTi-15V-3Cr-3Sn-3Al material joined by using the friction stir tool 101of FIG. 1.

This figure reveals that an orderly surface of bead has been obtained.In addition, no wear or deformation was found in the shoulder and pin ofthe friction stir tool. In this example, it was found by a visualinspection of the welding tool after joining and by other tests thatalmost no reaction was caused between the rotational tool made of theCo-based alloy and the titanium alloy.

The results mentioned above clearly show that the friction stir tool ofthe present invention is also effective for the friction stir welding ofthe titanium alloys.

Example 5

The results of the friction stir tool formed of a Co-based alloy havingthe composition Co-30Ni-9.5Al-7.8W-2Ta-10Cr-0.06B-0.6C are shown below.All units are by at. %. The amount of Co contained is about 40%. Thiswelding tool is denoted a welding tool 201. The welding tool 201 wasproduced in a manner similar to that of the welding tool 101. The volumefraction of the γ′ precipitate phase in the above welding tool 201 isabout 45%.

The results of determination of the solid solution forming temperatureof the γ′ precipitates by differential scanning calorimetry (DSC) areshown in Table 1. In this Table, in the case of the welding tool 101,that is, an alloy containing 40 at. % of Ni, the γ′ solid solutionforming temperature is 1163° C., while in the case of the welding tool201, that is, an alloy containing 30 at. % of Ni, the γ′ solid solutionforming temperature is 1094° C.

TABLE 1 Solid solution forming Solidus temperature of γ′ phasetemperature Welding tool 101 1163° C. 1339° C. Welding tool 201 1094° C.1358° C.

The results of the DSC in Table 1 show that the above welding tool 201of this example has a solid solution forming temperature of the γ′precipitates of 1094° C. This is because the amounts of Ni and Tacontained in the friction stir tool of this example are less than thosein the friction stir tool 101 of Example 1. The solid solution formingtemperatures of the γ′ phases in the welding tools 101 and 201 arehigher than the joining temperature of the steel material SS400 (1000°C.). Therefore, it is thought that the γ′ phase in the above weldingtool is stable even during joining.

In a manner similar to that of Example 1, the joining was conducted onthe SS400 iron-based material by the friction stir tool of this example.The friction stir tool of this example also had a shape similar to thatof the friction stir tool of Example 1. The joining conditions used werealso similar to those of Example 1. The joining distance was 360 mm, andjoining was repeated multiple times.

The results demonstrated that friction stir welding of the SS400material was also possible in this example.

Examples 6 to 12

The results of Examples 6 to 12 are shown in Table 2 along with those ofExamples 1 to 5.

Table 2 shows examples of applications of the welding tools 101 and 201to various joining materials.

It was confirmed that the welding tools 101 and 201 could be applied toan aluminum alloy having low melting point and stainless steels, Tialloys and a Zr alloy having high melting points without any problems.

The surfaces of the welding tools were observed after joining, and thecases where no damage in the welding tools, no deposition of weldingmaterials on their surfaces, and no defects in the joined parts werefound were evaluated ∘ (good).

The joining conditions applied to Examples 6 to 12 are as follows:

Example 6: N=800 rpm, V=200 mm/min, Example 7: N=200 rpm, V=100 mm/min,Example 8: N=100 rpm, V=50 mm/min, Example 9: N=200 rpm, V=100 mm/min,Example 10: N=200 rpm, V=100 mm/min, Example 11: N=100 rpm, V=50 mm/min,Example 12: N=200 rpm, V=100 mm/min, wherein N is the rotational speed,and V is the joining speed of the welding tool.

TABLE 2 6N01 SUS430 SUS304 (Al alloy) SS400 (AISI430) (AISI304) Pure TiTi64 Ti15333 Zircaloy Welding tool Example Example Example ExampleExample Example Example Example 101 6 (∘) 1 (∘) 7 (∘) 8 (∘) 2 (∘) 3 (∘)4 (∘) 9 (∘) Welding tool Example Example Example Example 201 5 (∘) 10(∘) 11 (∘) 12 (∘) PCBN Δ1 Δ2 Δ2 Δ2 x x x x (Comparative Example)

The γ′ volume fractions in Examples 1 to 12 are shown in Table 3.

The γ′ volume fractions in the welding tool after joining were describedas follows:

A: 10% or more but less than 30%, B: 30% or more but less than 50%, C:50% or more but less than 65%, D: 65% or more but 80% or less. Herein,for example, 10% or more but less than 30% means 10% or more and lessthan 30%.

TABLE 3 6N01 (Al alloy) SS400 SUS430 SUS304 Pure Ti Ti64 Ti15333Zircaloy Welding D C B A B C C B tool 101 Welding B A A B tool 201

As can be seen from Tables 2 and 3, the friction stir welding toolcomprising the Co-based alloy having a volume fraction of the γ′precipitate phase of 10% to 80% is obviously applicable to any joining.In addition, it can be seen that the γ′ volume fraction in the frictionstir welding tool is varied not only by the alloy composition of thewelding tool, but also by the type of the joining material and joiningconditions.

Comparative Example

Normally, when joining of the Ti alloy is conducted by using therotational tool made of PCBN, wear of the welding tool is severe, andthe surface of the joined part is also roughened. This is supposedlybecause N (nitrogen) contained in the rotational tool made of PCBNreacts with Ti (titanium) to form TiN (titanium nitride), which causesthe welding tool to be more likely to wear.

Thus, titanium is a metal having activity, and is often found to reactwith the friction stir tool during joining. The results of joiningactive metals by the welding tool made of PCBN are shown in Table 2, butthey were evaluated x (bad) since joining could not be conducted due tosevere wear of the welding tool.

When the rotational tool made of a ceramic material such as PCBN isrepeatedly used, cracks generated in the shoulder and pin and portionsof the welding tool chipping away are often found. It is therefore thepresent situation that the lifetime of a rotational tool made of PCBNcannot be predicted. This is presumably because ceramic materials arevulnerable to thermal shock.

The results of joining of materials such as SS400, SUS430 (AISI430), andSUS304 (AISI304) by using the rotational tool made of PCBN are shown inTable 2. They were evaluated Δ2 since sudden breakage of the weldingtool occurred during use. In contrast, it is supposed that abruptbreakage in the welding tool made of the Co-based alloy is unlikely tooccur since the Co-based alloy is a metallic material.

Furthermore, when the rotational tool made of PCBN was applied to thealuminum alloy (6N01), the joining material (aluminum) was deposited onthe surface of the welding tool, and the surface of the bead wasroughened in some cases. Such a result is indicated as Δ1 in Table 2.

It was found that when the above-mentioned friction stir tool (thestirring tool made of the Co alloy) is applied to the friction stirwelding of a structural portion of a high-melting point material,deformation by welding, spatters and residual stress are reduced. Thefriction stir welding using the friction stir tool of the presentinvention can be applied to structures such as panel members forautomobiles and pipes.

What is claimed is:
 1. A friction stir tool formed of a Co-based alloyhaving a γ phase with an A1 structure and a γ′ phase with an L1₂structure precipitated in the γ phase, the γ′ phase comprising (Co,X)₃(Al, W, Z), and the γ′ phase being contained in the γ phase in anamount of 10% to 80% in terms of volume fraction, the Co-based alloyincluding 30-50 at. % of Ni, and the γ′ phase thereof having a solidsolution forming temperature higher than 1,000° C., wherein the Co-basedalloy contains a precipitate selected from a μ phase, a Laves phase anda carbide, where X includes mainly Ni, and Z includes mainly Cr and Ta.2. The friction stir tool according to claim 1, being formed byprecision casting.
 3. The friction stir tool according to claim 1,wherein the Co-based alloy contains Co in an amount of 25 at. % or more.4. The friction stir tool according to claim 1, comprising a cylindricalshank, a shoulder formed in an end portion of the shank, and a pinformed in an end portion of the shoulder.
 5. The friction stir toolaccording to claim 4, being an integrally formed article of the Co-basedalloy produced by the precision casting.
 6. The friction stir toolaccording to claim 5, being a machined product of the Co-based alloymade of the integrally formed article.
 7. A friction stir weldingapparatus comprising the friction stir tool according to claim
 1. 8. Thefriction stir tool according to claim 1, wherein the Co-based alloyincludes about 10 at. % of Al, about 7.5 at. % of W, about 3 at. % ofTa, about 10 at. % of Cr, about 0.06 at. % of B, about 0.6 at. % of C,and at least 25 at. % of Co.
 9. The friction stir tool according toclaim 1, having a property of being capable of joining, by friction stirwelding, high-melting point materials selected from the group consistingof steel materials, titanium and alloys thereof, and zirconium andalloys thereof.
 10. The friction stir tool according to claim 1, beingheat-treated and air-cooled after the precision casting.
 11. Thefriction stir tool according to claim 1, wherein the cobalt-based alloycontains precipitates that include each of the μ phase, the Laves phaseand the carbide.